Protein-induced DNA distortion is essential for many normal cellular functions, including aspects of gene regulation, expression, recombination, and chromatin organization. DNA distortion is an energetically costly process that is intimately linked to the affinity and function of the protein-DNA complexes. A quantitative understanding of the energetics of protein-induced distortion lags far behind the level of structural information currently available. To a large extent, this derives from the difficulty of controlling DNA distortion and bending as experimental variables in complexes where both calorimetric and structural data are obtainable. The hyper-thermostable Sac7d-DNA complex is uniquely suited to serve as a model benchmark system for quantitative studies of the energetics of DNA bending and unwinding due to minor groove binding. In this proposal, we focus on the fact that it is possible to experimentally manipulate the level of DNA distortion in Sac7d-DNA complexes where structural changes and energetics can be followed. The goal is to define the distortion induced in DNA by Sac7d in solution, and to correlate direct measures of distortion with the energetics of the protein-DNA interaction. We will use fluorescence resonance energy transfer, DNA cyclization, NMR, and calorimetry to systematically investigate the role of specific amino acid residues, as well as the influence of DNA sequence and length on protein-induced distortion and binding energetics. In addition, we will investigate the influence of cellular conditions, including salt concentration, specific counter-ions, and osmolarity, on the magnitude of distortion and the associated energetics of binding and bending. This will provide the first direct structural and calorimetric measure of the linkage of DNA distortion to the energetics of protein-DNA binding. This is a basic research project which will provide a description of the energetics of an important type of DNA interaction that occurs in many disease-related protein-DNA complexes. Sac7d is a chromo-domain, a fold common in eukaryotic nuclear proteins that is also found in the DNA-binding domain of HIV-1 integrase. The protein binds to DNA via mechanisms similar to those observed in proteins with direct biomedical relevance. The results will enhance our ability to rationally control related protein-DNA binding interactions that are potential targets in pharmacology and therapeutics.